Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. standard Screen size and about 5 microns.
A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion thermal spray gun, the carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas, although other gases, such as hydrocarbons, may be used in certain situations.
The material alternatively may be fed into a heating zone in the form of a rod or wire. In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, where it is melted or at least heat-softened and atomized, usually by blast gas, and thence propelled in finely divided form onto the surface to be coated. The rod or wire may be conventionally formed as by drawing, or may be formed by sintering together a powder, or by bonding together the powder by means of an organic binder or other suitable binder which disintegrates in the heat of the heating zone, thereby releasing the powder to be sprayed in finely divided form. In other forms the wire may have a coating sheath of one component and a core of the others, or may be made by twisting strands of the components.
Coatings produced by thermal spraying alloys of nickel, iron, cobalt or combinations thereof as a base metal which contain, in the alloy, chromium and optionally aluminum and/or other elements are used to provide corrosion protection of metal components such as in gas turbine engines and boiler systems. Cobalt, for example, is used as either a base metal or an alloying element to improve high temperature creep and strength properties in cast and wrought superalloys. However, it is well known that cobalt is not classified as a oxidation resistant metal. Scaling and oxidation rates of unalloyed cobalt in air are many times those of nickel. The scaling and oxidation resistance of cobalt-base alloys at high temperature is largely a function of chromium content. As a result, cast or wrought parts fabricated of cobalt alloys frequently require special coatings for protection.
However, in the thermal spraying of such nickel, iron or cobalt alloys, the bond strength of the resultant coatings is often not satisfactory, even where the coatings are thermal sprayed using a plasma spray gun. Also, to obtain resistance to the corrosive conditions in the application, an alloying element such as yttrium or a rare earth metal is often added, but thermal spray powders or wires of such alloys are expensive to manufacture. Typical alloys of this type are disclosed, for example, in U.S. Pat. No. 4,313,760, and in U.K. patent application No. GB 2,028,378A published Mar. 5, 1980.
To achieve high density and improved bonding, alloy powders are plasma sprayed in a low pressure inert atmosphere chamber, an operation that is slow and costly and requires sophisticated vacuum and work handling equipment. There are similar and even more complex problems with vapor deposition which is an alternative coating method known in the field. Chambers also preclude deposition onto large components such as boilers.
Coatings having improved bond strength may be thermal sprayed using a composite powder formed of metals capable of reacting exothermically as described in U.S. Pat. No. 3,322,515. One such powder has a nickel core with about 5 percent by weight of fine particles of aluminum bonded to the surface thereof with an organic binder. The core may be alloyed with another metal such as chromium. Cobalt plus aluminum, and molybdenum plus aluminum are others of many exothermic pairs mentioned therein.
Several other patents teach improved clad powders to produce thermal sprayed coatings having good bond strength and the capability of being readily machined. One is U.S. Pat. No. 3,841,901 which discloses a powder of nickel, copper or iron core coated with fine particles of aluminum and molybdenum for producing a thermal sprayed coating which has good bond strength and can readily be machined. Similarly U.S. Pat. No. 4,181,525 teaches a thermal spray powder comprising particles having a core of nickel, iron, copper, cobalt or alloys thereof coated with a binder containing discreet particles of aluminum and substantially pure nickel, directed to coatings having improved machinability.
The composite powders disclosed in the above-mentioned patents are generally employed for bonding other coating materials to substrates such as steel, or for producing single step coatings for machine element applications requiring wear resistance and finishing capability. However, there has been only limited success with composite powders where corrosion resistance is required. The reasons are not well understood. In aqueous or moist environments, electrolytic problems appear to be associated with the heterogeneous nature of the coatings resulting from incomplete alloying of the cladding elements with the core during the thermal spraying process. However, protection is also lacking in dry, high temperature situations that are oxidizing or that involve sulfates and chlorides in either oxidizing or reducing conditions. If coatings contain any free nickel, as results from incomplete reaction or alloying during thermal sprayings the nickel-aluminum clad powder of U.S. Pat. No. 4,181,525, even where the powder has a nickel chromium alloy core, the coatings are especially vulnerable to attack in certain corrosive conditions. The attack is not only in the coating material but in the interface, weakening the bond and causing coatings to spall.
Chromium is used as an alloying element in a powder core to improve corrosion resistance of coatings of a thermal spray powder in which the core is clad with aluminum. However, the chromium additions have proven to reduce the bond strength of the thermal sprayed coating. For example, aluminum clad nickel-chromium alloy provides lower bond strength than aluminum clad nickel.
As taught in U.S. Pat. No. 3,322,515, for bonding purposes iron is not of itself a satisfactory component in a composite with aluminum, and iron-chromium alloy clad only with aluminum has especially poor bonding when thermal sprayed.
Thus, although composite thermal spray powders are known and available which may produce satisfactory bond strengths, higher tenacity is desired for corrosive environments, and the coatings produced from such powders are particularly lacking in sufficient corrosion resistance. On the other hand, the known alloy powders used for thermal spraying coatings for high temperature corrosion protection lack sufficient bond strength.
In view of the foregoing, a primary object of the present invention is to provide a novel thermal spray material for producing tenacious metallic coatings characterized by both high bond strength and hot corrosion resistance.
A further object of this invention is to provide an improved thermal spray process for producing tenacious metallic coating characterized by both high bond strength and hot corrosion resistance.